In APCs, antigen processing compartments often have the phenotypic characteristics of multivesicular bodies (MVB). In studies examining MHC-II trafficking to antigen processing compartments we found that activated (but not resting) B cells secrete significant amounts of their total pool of pMHC-II on MVB-derived vesicles termed exosomes. Surprisingly, it was predominantly Ii-free surface pMHC-II that internalized and trafficked to these MVB, highlighting a previously unrecognized transport pathway followed by pMHC-II. We also found that interaction of antigen-loaded B cells with antigen-specific T cells stimulates exosome release from B cells, and these exosomes in turn can stimulate primed (but not nave) T cells to proliferate. Our results support a model that T cell stimulated exosome release from activated B cells serves to augment T cell responses. The movement of proteins and lipids from one intracellular compartment to another is carried out by a well-orchestrated process of transport vesicle formation, vesicle docking with a target compartment, and finally vesicle-target membrane fusion. The proteins that catalyze membrane fusion are termed SNAREs. We have been investigating the role of distinct SNARE isoforms in regulating secretory granule exocytosis from immune cells using a variety of SNARE knock-out mice and have found that VAMP-8, but not VAMP-2 or VAMP-3, regulates mast cell secretory granule exocytosis. Surprisingly, VAMP-8-deletion only affects serotonin exocytosis but not histamine or TNF-alpha exocytosis, showing for the first time that secretory granule heterogeneity exists in mast cells. We are currently using these mice to examine the role of specific SNARE proteins in regulated exocytosis from other immune cells.